A wide variety of absorption processes have been proposed for removing acid gases such as, for example, carbon dioxide, hydrogen sulfide, sulfur dioxide, sulfur trioxide, carbon disulfide, hydrogen cyanide, and carbonyl sulfide from process gas streams using absorbents comprising amines.
Such absorption processes typically involve passing the process gas stream containing one or more of the acid gases to an absorption zone wherein it is contacted with a lean solvent comprising the amine absorbent. A product gas stream, depleted in the acid gases relative to the process gas stream, is withdrawn from the absorption zone as a product. A rich solvent stream comprising the amine absorbent and the absorbed acid gases is also withdrawn from the absorption zone and passed to a regeneration zone, e.g., a steam stripping column, wherein the absorbed acid gases are desorbed from the solvent to provide a tail gas stream comprising the acid gases and the lean solvent stream hereinbefore described.
A common problem in such acid gas absorption processes is that heat stable salts of the amine are often formed during one or both of the absorption and regeneration steps as a by-product. Heat stable salts can be formed, for example, when strong acids such as hydrochloric acid or sulfuric acid are present in the process gas. Heat stable salts can also be formed when sulfite anions are oxidized to sulfate anions. Typical ions which form heat stable salts, i.e., heat stable anions, include, for example, sulfate anions, thiosulfate anions, polythionate anions, thiocyanate ions, acetate ions, formate ions and oxylate ions.
Heat stable salts generally do not have absorption capacity for the acid gases and are not regenerable under the conditions of the process. Therefore, the level of heat stable salts needs to be controlled in order to retain an adequate degree of absorption capacity for the acid gases.
Electrodialysis has been proposed as a method for removing heat stable salts from amine containing streams. In a typical electrodialysis process, caustic, e.g., sodium hydroxide, is added to the stream containing the heat stable amine salt in order to dissociate the heat stable anion from the heat stable salt and provide an amine in free base form and a simple heat stable salt, e.g., sodium sulfate. The simple salt is then separated by electrodialysis wherein the charged ions permeate through anion- and cation-selective membranes. The amine, which is non-ionic, does not permeate through the membranes and is discharged from the electrodialysis zone as a product. Often, conventional electrodialysis processes operate in a batch mode wherein the process streams are recirculated until the desired amount of heat stable salts is removed.
Certain problems can result from the use of electrodialysis processes such as described above. For example, since the amine product from the electrodialysis zone is provided in free base form, it can have excessive volatility which can lead to solvent losses during absorption. In addition, since the process is a batch process, the pH and ionic strength within the compartments of the electrolysis zone vary with the discontinuous operation. As a result, the membranes in the electrodialysis zone often experience shrinking and swelling and, ultimately, are subject to mechanical failure. Moreover, to the extent that the amine is not converted to free base form in the caustic treatment step, there can be substantial losses of the amine due to permeation through the membranes in the electrodialysis zone. In addition, conventional electrodialysis requires a source of caustic.